There is a strong demand in the market to have a practical solution for a broadband high-isolation circulator enabling Simultaneous Transmit and Receive (STAR) systems for emerging wireless communication and radar applications.
The electromagnetic spectrum is extremely valuable to users. In a recent auction of spectrum for wireless services, companies bid substantial amounts for access to the wireless spectrum for advanced wireless services. STAR (simultaneous transmit and receive) systems can double the spectral efficiency by allowing simultaneous data transmission and reception at the same frequency band at the same time. Applications for use of STAR systems include, but are not limited to, 5G-and-beyond wireless communications, radar in autonomous vehicles, in-band full-duplex relay, self-organizing networks, device discovery in device-to-device communications, jamming mitigation, and imaging.
A major technical challenge in implementing STAR systems is self-interference. Self-interference results in signals that are being transmitted also being directly coupled to the radio frequency (RF) receiving chain of the device. The coupling of the transmitted signals into the receiving portions of the device can make the RF front-end of the device insensitive to incoming signals from other radio transmitters and/or damage the RF front-end.
A possible solution may be to utilize a high-isolation circulator (a circulator having a greater than 45 dB isolation between transmitting and receiving operations over frequencies of interest), but typical compact circulators that are available in the market only offer about 20 dB of isolation between the ports. The low-isolation circulators may be used along with advanced analog and digital cancellation techniques, but this requires an increase of RF components and an increase in silicon area on the chip, which translates to higher power consumption and increased area in the RF front-end and in the radio chips. Some high-end circulators offer 35 dB or higher of isolation, but they often have extremely-narrow isolation bandwidth. In addition, return loss at each port of the circulator is required to be higher than the isolation level provided by the circulator. In other words, if a circulator having a 45 dB isolation level is used, the return loss at each port should be equal to or greater than 45 dB. Achieving these values can be extremely challenging in a practical system implementation.
Other solutions have been proposed in the literature, but these also suffer from low duplex isolation levels, narrow duplex-isolation bandwidths, a lack of channel reciprocity support, a large physical size, high insertion loss, and/or lack of high power handling capability.
The proposed solutions include among others (a) use of a conventional circulator, (b) use of two orthogonal antennas, (c) use of an antenna cancellation technique, (d) use of a directional coupler with a reflective load, (e) use of a loop circulator connecting three circulators, and (f) use of a magnet-less circulator. Each has its shortcomings.